Processes for recovering hydrochloric acid from waste pickle liquor are known as exemplified by my prior U.S. Pat. No. 4,222,997. The regeneration of hydrochloric acid from waste pickle liquor still remains a problem, however, because the energy costs necessary to operate a recovery process continue to increase. Since the environmental disposal of waste hydrochloric acid pickle liquor also remains a critical factor, new and improved recovery processes are desirable and necessary.
Several of the reasons for recovering hydrochloric acid from waste pickle liquor have been given in my prior patent. Further investigations have shown that it is desirable to produce a stronger acid than that obtained from the process of my earlier patent, namely, greater than 34% HCL. It is also desirable that the recovery process be improved for controlling the quality of the hydrochloric acid as well as varying the concentration thereof. It has been found that a much wider range of hydrochloric acid concentrations can be produced with improved quality.
Waste pickle liquor typically contains 14-25% ferrous chloride, 3-7% hydrogen chloride, and several impurities, including inhibitor residues and trace elements which are picked up during the pickling process by the action of the acid on the steel being pickled. These impurities greatly reduce the use of the ferrous sulfate monohydrate by-product that is produced from the recovery process described in my prior patent. Since high quality ferrous sulfate is used as a food additive and in many commercial chemical processes, it is desirable that a recovery process be provided which will materially improve the quality of the ferrous sulfate monohydrate by-product produced. Further, it is desirable to form hydrates of ferrous sulfate other than the monohydrate sulfate since they are also widely used.
Thus, it is an object of the present invention to recover a stronger hydrochloric acid from the waste pickle liquor and to mix that stronger acid with weaker acid from another part of the process thereby producing controlled variable concentrations of hydrochloric acid. It is another object of the present invention to produce high quality ferrous sulfate crystals in several hydrate forms. A further object of the invention is to reduce the amount of vapor removed from the sulfuric acid reactor by having the primary concentration done at a lower temperature in a ferrous chloride concentrator crystallizer.